1. Field of the Invention
The invention relates to a wheel bearing device.
2. Description of the Related Art
A wheel bearing device that supports a wheel of a vehicle includes an outer ring, an inner ring, rolling elements, and a cage, as described in, for example, Japanese Patent Application Publication No. 2009-250297 (JP 2009-250297 A). The outer ring is fixed to a vehicle side fixing member disposed on the inner side (inboard side) in the vehicle lateral direction (hereinafter, referred to as “vehicle inner side”). The inner ring is fitted to a wheel disposed on the outer side (outboard side) in the vehicle lateral direction (hereinafter, referred to as “vehicle outer side”). The rolling elements are disposed in two rows arranged in the axial direction of the wheel bearing device, at positions between the inner ring and the outer ring. The cage retains the rolling elements arranged in the circumferential direction of the wheel bearing device.
For example, when the wheel equipped with a tire hits a curb, which is disposed along a road and forms an edge of a sidewalk, from the side while the vehicle is travelling, an impact moment load around the axis of the wheel bearing device is applied via the tire to a portion of the wheel bearing device, which is located vertically below the axis. As a result, the moment load turns into a rolling element load that is applied largely to a portion of the wheel bearing device, which is located vertically below the axis, on the vehicle outer side, and applied largely to a portion of the wheel bearing device, which is located vertically above the axis, on the vehicle inner side. In the wheel bearing device, indentations are formed in both the raceway surfaces of inner and outer rings, to which an excessive rolling element load is applied, and these indentations contribute to noise generation.
As a wheel bearing device with an improved rigidity against a moment load, there is a wheel bearing device of a so-called different-sized PCD type, as described in Japanese Patent Application Publication No. 2004-108449 (JP 2004-108449 A). Specifically, a first rolling element group disposed on the vehicle outer side has a larger pitch circle diameter (pitch diameter of ball set) PCD and a larger number of balls than those of a second rolling element group disposed on the vehicle inner side. In this wheel bearing device, the PCD is set larger on the vehicle outer side, on which a rolling element load is likely to be larger due to the closeness to the wheel and there is more flexibility in layout, than on the vehicle inner side on which the flexibility in layout is limited due to the influence of the vehicle side fixing member. In this way, the static load rating on the vehicle outer side is increased, and formation of indentations in the raceway surfaces is restricted.
If the wheel bearing device described in JP 2004-108449 A receives an excessive impact moment load beyond a permissible value of the static load rating, indentations may occur in both the raceway surfaces on the vehicle inner side and the vehicle outer side at the same time. In the bearing device in which indentations are formed, noise is generated as the rolling elements sequentially roll on the indentations formed in the raceway surfaces. In the wheel bearing device in JP 2004-108449 A, because the first rolling element group and the second rolling element group are different from each other in the PCD and the number of rolling elements, the revolution speed of the rolling elements is different between the vehicle inner side and the vehicle outer side. This results in a slight difference in cycle of noise generation between the vehicle inner side and the vehicle outer side. The slight difference in the cycle, that is, frequency of noise generation causes a difference in frequency, which generates a beat. Such a beat gives an occupant an uncomfortable feeling. Thus, it is necessary to increase the static load rating on the vehicle outer side and to reduce a beat due to indentations in a wheel bearing device of a different-sized PCD type.